How can an AI system develop its domain knowledge? Is there more than just Machine Learning?
Well, we are talking about a system (a machine) which develops knowledge (learns), so it is kind of difficult for such a technique to not fall within machine learning.

But you could argue that inference engines which work on a graph based knowledge database to derive new propositions or probabilities are not part of machine learning. Of course in that case part of the knowledge is not acquired at all, but rather entered by the developers. 

I'm still reading up on this, but my impression is that these knowledge databases and inference engines became rather popular in the nineties and many AGI-researchers today still work in that direction.